A print substrate of the type mentioned above is described in DE-A-38 41 199. This is a matt-coated printing paper for offset printing or rotogravure printing whose gloss, according to the examples contained in that text, ranges from 15% to 38% as measured at an angle of 75.degree. according to Lehmann. The proportion of ungelatinized starch granules in the known printing paper ranges from 10 to 65 percent by weight with respect to the total solids content of the coat. In addition to the starch granules, calcium carbonate is also contained as mineral pigment, preferably in an amount ranging from 25 to 70 percent by weight with respect to the solids content of the coat. Although a proportion of other mineral pigments--apart from calcium carbonate--amounting to 63 percent by weight with reference to the total coating pigment can be calculated from this data based on a binder proportion of 5 percent by weight, it is expressly stated that the maximum proportion of other pigments, e.g. coating china clay or kaolin, is advisably not more than 15 percent by weight with reference to the solids content of the coat.
Ungelatinized starch granules were also already suggested in DE-A-25 01 684 for the production of embossable, printable and washable raw or base wallpaper. According to this suggestion, 10 to 100 percent by weight of the total pigment content is added in the form of ungelatinized starch so that, according to the stricter teaching, the presence of inorganic pigment can also be dispensed with. To achieve sufficient washability, the binder proportion is 15 to 100 percent by weight, preferably 50 to 60 percent by weight, with respect to the total pigment content, where it is suggested that calcium silicate be added as mineral pigment.
DE-B-12 21 893 describes the production of a coated printing paper in which starch is rendered partially cold-soluble by mechanical treatment in a pug mill so that when drying is effected subsequent to the coating process it will still be dissolved by the evaporating water of the coating compound and, in dissolved or gelatinized form, contributes, as binder, to the fixation of the pigments, e.g., chalk, in a manner known per se. The produced papers have a confirmed high gloss, but this older suggestion is not capable of fully satisfying the requirements for modern printing paper.
DE-A-26 05 575 is directed to a coated paper, in particular for paper currency and security documents, whose surface is treated with a mixture of latex and ungelatinized starch granules as filler. Conventional dispersing agents, natural binders and common filler materials are mentioned as other additives, although no specific quantities are mentioned. The amount of ungelatinized starch added is in the range of 0.25 to 25 parts by weight for 1 part by weight of the flexible polymers consisting of latex. The produced paper has a matt appearance and should not be excessively calendered so as not to impair the properties of the paper such as printability, storage stability and resistance to soaking and changes in moisture content.
It will be gathered from the references discussed above that ungelatinized starch granules have previously been mentioned in relation to the production of special papers such as bank note paper, embossed wallpapers and for matt printing paper. However, for the substantially more important field of printing paper with standard glossiness as used, for example, as print substrates for illustrated magazines, mail order catalogs, advertisement brochures, etc., where these print substrates are usually coated on both sides, ungelatinized starch granules have not previously been mentioned as additives for the coat absorbing the printing ink. However, there is an urgent need to improve the printability of such print substrates, in particular lightweight coated papers, so-called LWC paper, particularly also because of the need for a steady reduction in the grammage of raw or base paper and in the coat weight to be applied in order to lower the cost of this paper. Thus, the largest possible printable surface must be provided while reducing the weight of the paper. Since, as is well known, appearance of the printed image, particularly in paper to be printed by rotogravure printing, substantially depends on the surface of the paper when this paper is coated, i.e. on the smoothness and evenness of the coat, this paper must be calendered, usually by using so-called supercalendering. In spite of the most careful selection of the raw components to be used, in particular the coating pigments and binders, the strict requirements with regard to printing quality in rotogravure papers can not always be met. More particularly, the problem consists in further reducing the number of missing dots. By missing dots is meant unprinted defective locations which occur due to slight unevenness in the surface of the paper because the web to be printed does not accurately contact the rotogravure form, so that the printing ink located in the cells of the rotogravure form is not sucked out of these cells and voids occur in the printed image due to the absence of printing ink.